Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). Direct injection allows higher fuel efficiency and higher power output to be achieved in addition to better enabling the charge cooling effect of the injected fuel. Direct injected engines, however, also generate more particulate matter emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. Similarly, the injected fuel may encounter less turbulence when flowing through the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions. In addition, particulate matter generated in gasoline direct injected engines may be finer than their diesel engine counterparts.
In some engine systems, a wall flow filter (or closed filter) may be used to filter the particulate matter from the exhaust. However, such wall flow filters may require periodic active regeneration which may adversely affect engine operation. In addition, such wall flow filters may suffer from very high back pressures, especially when coated with a catalyst. The high filter back pressure can degrade the performance of a turbocharger by reducing a pressure drop across the turbocharger. As such, this may adversely affect downsized direct injected engines that are turbocharged to provide power that is comparable to larger displacement conventional engines.
Another example of a particulate matter (PM) filter used to capture soot generated by a gasoline engine is shown by Wei et al. in US 2009/0193796. Therein, an open PM filter is included in the emission treatment system either uncoated, or coated with a suitable catalyst (e.g., a TWC catalyst) that facilitates passive regeneration of the filter.
However, the inventors herein have recognized a potential issue with such systems. As one example, the amount of catalyst loading on the filter may be limited due to backpressure constraints. The limited catalyst loading may not enable sufficient regeneration of the filter. As such, incomplete filter regeneration may reduce filter performance and degrade exhaust emissions. In addition, the limited catalyst loading may result in increased secondary emissions from the filter, such as CO from incomplete soot combustion and hydrocarbon slip. If the catalyst was on the filter to promote other reactions such as NOx reduction there could be a reduction in activity if the catalyst is blocked by soot or the activity could be limited by the smaller amount of allowable catalyst loading due to backpressure concerns.
Thus, in one example, some of the above issues may be addressed by a method comprising, during stoichiometric engine operation, flowing exhaust gas over a filter having a catalyst coating. Then, during selected conditions, shifting engine operation to leaner than stoichiometry including a first shorter and more lean phase followed by a second longer and less lean phase, the lean operation adjusted based on a catalyst oxygen content and a filter particulate matter load. In this way, particulate matter emissions from a direct injected engine can be reduced.
In one example, while a vehicle engine is operating at stoichiometry, engine exhaust may be flown over an open particulate matter filter to collect exhaust PMs, or soot. The filter may be a layered filter including at least a first catalytic coating of an oxygen storage catalyst layered on the filter substrate, and optionally a second catalytic coating (e.g., of a three-way catalyst) layered on top of the first catalytic coating. Alternatively, the catalytic coating may include a single layer of a three-way catalyst having a very high oxygen-storage catalyst content. During selected conditions, such as when the vehicle is decelerating, a lean engine operation may be opportunistically performed to passively regenerate the filter. In particular, the engine may be operated leaner than stoichiometry over a first lean phase immediately followed by a second lean phase wherein the first lean phase is shorter but more lean while the second lean phase is longer but less lean. The first lean phase may be adjusted based on a catalyst oxygen content so as to oxidize (or reoxidize) the oxygen storage catalyst of the first catalytic coating using the exhaust oxygen. In addition, some of the PMs stored on the filter may be directly oxidized. The second lean phase may then be adjusted based on the filter load as well as the first lean phase to enable the oxidized oxygen storage catalyst to complete extensive combustion of the remaining PMs stored on the filter. Overall emissions control is achieved.
As such, the periodic lean operation may be performed to passively regenerate the filter while the filter load is lower than a threshold, and reduce the likelihood of the filter load exceeding the threshold. However, if the filter load exceeds the threshold, the filter may be actively regenerated wherein stored PMs are burned to reduce backpressure caused by soot retained on the filter. For example, the active filter regeneration can occur via chemical means when excess fuel is passed over an upstream catalyst containing precious metals, causing an exotherm that actively heats the downstream filter. The fuel could be added as a late injection within the engine cylinder or via a secondary injector directly into the exhaust pipe. As still another option, the filter may be heated via external means, such as electrical heat. In one example, in the absence of any catalyst, in air, regeneration of the filter may require a temperature of around 550° C. to burn the stored soot.
In this way, a periodic leaning of engine operation may be used for sufficient passive regeneration of a PM filter. By using a first lean phase to oxidize an oxygen storage component coated on the filter, the catalyst coating may be activated while at least some PMs are burnt. By following the first lean phase with a second lean phase that uses the activated catalytic coating to oxidize the remaining PMs, the filter may be substantially completely regenerated. By reducing the need for active regeneration of the filter, wherein an additional amount of fuel is used to increase the temperature of the filter and burn off the stored soot, over-temperature related component degradation may be reduced while also improving the fuel economy of the vehicle.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.